1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machines, laser printers and the like, and more specifically relates to an image forming apparatus capable of maintaining image density stability against varying humidity conditions.
2. Description of the Related Arts
In the aforesaid type of copying machine and the like, image density will vary depending upon the external environment. More specifically, when the developing material is allowed to sit undisturbed over a long period of time under high moisture conditions (for example, at night), a large quantity of toner will adhere to the surface of the photosensitive drum upon the initial developing due to a reduction in the amount of charge maintained by the toner as a result of the gradual loss of said charge during storage. As a result, the print density becomes too dense, thereby causing the so-called fog phenomenon.
Japanese Unexamined Patent Application No 58-33271 proposes a method for correcting a standard T/C (ratio of toner content to carrier content) based in the output of a humidity sensor that detects the humidity during machine operation.
The amount of electrical charge on the toner is changed in conjunction with variations in humidity. When the humidity changes in the increasing direction, the toner charge is reduced in proportion to the increase in humidity, but the toner charge is not likewise increased proportionately when the humidity changes in the decreasing direction. More specifically, the toner charge is gradually increased after a certain time period has elapsed.
Thus, when the standard T/C is corrected based on the humidity during the copying operation by the aforesaid conventional method, problems seldom arise when the humidity changes in the increasing direction, although when the humidity changes in the decreasing direction, the standard T/C ratio becomes larger before the toner charge increases. The aforesaid situation results in excessively dense print density and like disadvantages.